Cultured mouse fibroblasts which are malignantly transformed or treated with TPA or growth factors such as PDGF synthesize and secrete the 39,000 Mr precursor to cathepsin L (also called MEP, for major excreted protein) in large amounts. Cathepsin L is an active acid protease whether or not it is glycosylated. Transformation, TPA and PDGF stimulate transcription of cathepsin L. we have cloned a functional procathepsin L gene from the mouse and have identified and sequenced the 5' flanking region which acts as a promoter in CAT constructions. Regulated transcription requires sequences in the first two exons and introns acting in concert with upstream promoter elements. A human procathepsin L cDNA has also been cloned and sequenced. A study of the activity of recombinant human procathepsin L produced in bacteria indicates that both the "pro" and the carboxy-terminal part of the protein are needed for full activity. The "pro" piece appears to be involved in proper folding of the enzyme, while a carboxyterminal segment cysteine is also essential for activity, presumably because it allows formation of a disulfide bridge with the large catalytic subunit. Polyclonal and monoclonal antibodies to human cathepsin L have been prepared using the recombinant human antigen. Using the human procathepsin L cDNA probe, we have shown expression of procathepsin L mRNA in all human tissues and cell lines tested, with increased expression in renal cancer and squamous cell carcinoma of the lung.